1. Field of the Invention
The present invention relates to a substrate for a magnetic disk useful for a hard disc or a flexible disk.
More particularly, it relates to a substrate for a magnetic disk wherein a continuous thin metal layer is used and a process for its production.
2. Discussion of Background
A magnetic recording medium wherein a thin magnetic metal film of e.g. a Co-Cr alloy or a Co-Ni alloy is used, is expected to be a medium having a high recording density by virtue of its high saturation magnetic density and surface smoothness. Various studies are being made for its practical application. In particular, Co-Cr having vertical magnetic recording characteristics has been developed to have a linear recording density as high as 200 KBPI. Further, it has also been reported that Co-Ni provides a linear recording density of 70 KBPI when used as a horizontal magnetic recording medium of a thin film at a level of 500 .ANG..
Both of the above-mentioned recording media are far superior to presently widely used .gamma.-Fe.sub.2 O.sub.3 coating type media having a linear recording density of from 15 to 20 KBPI.
However, continuous thin film media having such excellent magnetic recording characteristics have not yet been widely practically used. One of the factors which hinder the practical application, is a problem in the sliding characteristics between the magnetic head and the medium.
Namely, a continuous thin metal film medium has a smooth surface, whereby, in the case of a hard disk, magnetic head crash is likely to take place due to sticking of the medium to the magnetic head, and in the case of a flexible disk, magnetic head crash is likely to take place due to an increase of the friction coefficient for the sliding operation because of a difficulty in maintaining a lubricant.
In order to solve such problems, it has been attempted to apply mechanical texture treatment i.e. surface treatment to form scratch marks of linear groove shape on the substrate surface by means of sandpaper or the like. FIG. 13 is a differential interference microscopic photograph (400 magnifications) of the surface texture of the substrate after the mechanical texture treatment. FIG. 14 is an enlarged perspective view of a portion of 248.times.248 .mu.m of the same texture as measured by a laser interference type roughness meter.
Such mechanical texture treatment is effective to some extent. However, in a medium wherein a high recording density is required, bit errors tend to increase depending upon the density and the depth of the texture (linear grooves), and it is difficult to control such density and depth at a mass production level. Namely, when the track width of the magnetic recording is narrow at a level of from 10 to 20 .mu.m, the width of the texture is required to be 1/10 of the track width. Otherwise, the modulation (the variation in the reproduction output when the recording current is varied) tends to exceed 10%. On the other hand, if the depth of the texture exceeds 200 .ANG., the head output tends to decrease by at least 10%, whereby the modulation increases.
Even when a texture satisfying such requirements were obtained, if the number of grooves per unit area is small, a problem of sticking would be likely to result since the sliding area of the magnetic head is as large as 0.5 mm.times.4 mm.
Further, due to irregular roughness such Agnail formed by mechanical texture treatment, the distance between the head and the magnetic recording medium can not adequately be reduced, whereby it is difficult to increase the recording density because of the spacing loss. Especially in the case of a hard disk, irregular roughness produces a dust during the CSS operation, which in turn adheres to the head and the medium to cause head crash.
To solve the above problems, the present inventors have proposed in U.S. patent application Ser. No. 07/356,873 a substrate for obtaining a magnetic disk having sliding characteristics free from magnetic head crash by packing pores of anodized aluminum with a material different in the physicochemical properties from the anodized aluminum on the basis of the regularity of pores of anodized aluminum i.e. the pore diameters and pores distances (cell diameters) being very small and the pore distribution being uniform, and subjecting it to etching treatment by utilizing the difference in the physicochemical properties between the anodized aluminum and the packing material, to form uniform two-dimensional fine roughness on the substrate surface, and a process for its production.